Thanks for the kind words. Yes, this is a case where I am certain that the box pieces are all original to the machine based on the ownership history and condition of the unit when I received it. And, the list looks good to me! All of that is consistent with the CIB machines I have in my collection.
This thread and
@atakioki's work above inspired me to look on the underside of my machines' accessories covers, and lo and behold there were the labels:

Left:
Power Mac G4 Dual 1.25 GHz (FW800); Right:
Power Mac G4 Quicksilver 867 MHz
For the Quicksilver I read these numbers as:
658-0232 ??
600-8671 Blank DVD-R Disc
600-8671 Blank DVD-R Disc
600-9427 Power Mac G4 Media (CD packet) **
600-9426 Power Mac G4 Print (documentation packet) **
590-1122 RJ-11 Modem Cable
590-2286 4-pin-to-6-pin FireWire Cable
590-2299 USB Extension Cable **
**Part numbers that are verified off the parts with my system.
What's weird here is that there is only one remaining mystery number but the keyboard, mouse, and power cable are all unaccounted for. Maybe it's for the cable and the kbd/mouse didn't get one? Not sure.
I've learned here that I am actually missing the modem cable for this system, so I'll be ordering that off eBay stat! I also hadn't included the DVD-R discs in the accessories photos for this machine as I wasn't sure if they were original to the machine, but now I know they were.
For the MDD I read these numbers as:
590-5180 Power Cable
591-0079 USB Extension Cable **
603-0599 Speaker Grille **
603-2120 DVI to VGA Adapter **
657-0210 Apple Pro Mouse (black) **
602-5222 ??
591-0075 RJ-11 Modem Cable **
603-2200 Power Mac G4 Print (documentation packet) **
603-2495 Blank DVD-R Disc
658-0235 ??
603-2795 Power Mac G4 Media (CD packet)
** Part numbers that are verified off the parts with my system.
One of the two remaining items must be the keyboard; I am not sure what the last item would be.
If anyone is able to confirm what the remaining parts/items are for either of these systems, would be keen to hear it!